Superconducting film and method of manufacturing the same

ABSTRACT

The present invention relates to a superconducting film having a substrate and a superconductor layer formed on the substrate, in which nano grooves are formed parallel to a current flowing direction on a substrate surface on which the superconductor layer is formed and two-dimensional crystal defects are introduced in the superconductor layer on the nano grooves, and a method of manufacturing this superconducting film. A superconducting film of the invention, which is obtained at low cost and has very high Jc, is useful in applications such as cables, magnets, shields, current limiters, microwave devices, and semifinished products of these articles.

TECHNICAL FIELD

The present invention relates to superconducting thin and thick filmshaving a high critical current density in a magnetic field which can beused in the filed of cables, magnets, shields, current limiters,microwave devices, semifinished products of these articles and the likeand a method of manufacturing the films.

BACKGROUND ART

When a magnetic field of not less than a lower critical magnetic fieldH_(cl) is applied to a superconductor, quantized flux lines(φ₀=2.07×10⁻¹⁵ Wb) are formed and penetrate into the superconductor.When a current is caused to flow in this state, the Lorentz force actson the quantized flux lines. When these quantized flux lines begin tomove, a voltage is generated and the superconducting state is broken. Itis known that, for example, in a superconducting film formed from ahigh-temperature oxide superconductor YBa₂Cu₃O_(7−x) (YBCO), dot-likedefects such as naturally introduced oxygen deficiency and fineimpurities function as pinning centers of quantized flux lines.Furthermore, it is known that one-dimensional defects such asdislocations and two-dimensional defects such as crystal grainboundaries function also as pinning centers. In this case of YBCO, it isimportant that these crystal defects be present perpendicular to thefilm plane. In general, YBCO-based high temperature superconductors arematerials which have high crystal anisotropy and, therefore, when amagnetic field is applied parallel to the c-axis of a crystal, Jc tendsto decrease greatly compared to a case where a magnetic field is appliedperpendicularly to the c-axis. A usually used YBCO thin film is formedso that the c-axis is perpendicular to the film plane (surface), and,therefore, Jc decreases greatly when a magnetic field is appliedperpendicularly to the film plane (surface). When a superconducting tapefabricated from a YBCO thin film is used to form a coil, magnetic fieldcomponents of low Jc parallel to the c-axis govern coil properties,since a parallel magnetic field and a perpendicular magnetic field areapplied to the tape. However, when one-dimensional defects or crystalgrain boundaries are present in a direction parallel to the c-axis, theybecome pinning centers of quantized flux lines and Jc in this directionis improved. Therefore, the crystal orientation of one-dimensionaldefects or crystal grain boundaries is very important for improvement incoil properties. In contrast, this does not apply to dot-like defectsetc. since they are isotropic.

The relationship between the dislocation density in a YBCO film and Jchas been reported by Dam (see B. Dam et al., Nature, Vol. 399, p439,1999). According to the report, dislocation densities of 10 μm⁻² to 100μm⁻² can be obtained by changing film forming conditions in various waysand Jc increases with increasing dislocation density, although it isdifficult to control the density per unit area of dislocations which arenaturally introduced during film growth.

Crystal grain boundaries function not only as pinning centers, but alsoas barriers of superconducting currents. In fact, in a high temperaturesuperconducting film of YBCO etc., Jc is very small in a grain boundaryhaving a large inclination (the angle of a grain boundary to a normalline of the ab-plane of YBCO), but large Jc is maintained when theinclination is low. A low angle grain boundary can be regarded as adislocation array. Although a dislocation is an insulator(non-superconductor), in a low angle grain boundary having a largespacing between dislocations, a strongly-coupled superconducting partexists between dislocations and a large superconducting current flowsthrough the low angle grain boundary. However, when the inclinationincreases and the strains of dislocations begin to overlap, the currentbecomes less likely to flow. If boundary planes are parallel to acurrent flowing direction, they become very effective pinning centers.In general, however, boundary planes exist randomly, it is difficult tocontrol Jc by controlling the inclination of boundary planes.

On the other hand, fine precipitates having a size close to the coherentlength of the superconductor are also effective as pinning centers.Furthermore, artificial defects introduced by lithography and columnarcrystal defects introduced by electron beam irradiation and heavy ionirradiation also become pinning centers. There is a possibility thatdesired pinning centers can be introduced by lithography in a film.

In a case where electron beam exposure is used, there is a report thatthe pin diameter can be decreased to the order of 10 to 20 nm, althoughit has not been able to reduce the pin diameter to the nano level. Also,the pin spacing can be adjusted to the same extent. Examples ofmeasurement experiment of critical currents show that some peaks appearin superconducting properties in magnetic field depending on therelationship between quantized flux lines and pin arrangement (see J. Y.Lin et al., Phys. Rev. B54, R12712, 1996). Although this method iseffective in artificial pin introduction, from a practical viewpoint,the throughput is low and the cost is too high for a large areafabrication and for wire fabrication. In heavy ion irradiation and thelike, columnar defects are formed in superconducting crystals and thisis effective in improving Jc. However, the equipment cost and the costof ion acceleration are very high. Furthermore, in some cases materialsare radioactivated and hence these methods are not practical.

In order to introduce crystal defects such as dislocations in a film,there is also available a method by which island-like crystals such asnano dots are formed on a substrate surface and a superconducting filmis formed on the island-like crystals. There is an exemplary reportthat, in this case, Jc is improved by forming nano dots of Ag on asubstrate (see A. Crisan et al., Appl. Phys. Lett., Vol. 79, p4547,2001). A literature of Dam suggests a principle that when fineprecipitates exist in the process of growth of a film on a substrate,the continuity of film growth is lost on the fine precipitates,resulting in crystal defects, dislocations and grain boundaries (see B.Dam et al., Physica C341-348, p2327, 2000). According to thesetechniques, however, the arrangement of introduced defects is random andthe pinning force is averaged. Therefore, these techniques have theirlimits in drastically improving Jc.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a structure of idealpinning centers in a superconducting film which is formed on a substrateand a method of introducing the pinning centers. Another object of theinvention is to provide, at low cost, a technique for increasing Jcwhich can be applied to all Type II superconductors of film shape.

A superconducting film of the first embodiment of the invention has asubstrate and a superconductor layer formed on the substrate, in whichnano grooves are formed parallel to a current flowing direction on asubstrate surface on which the superconductor layer is formed andtwo-dimensional crystal defects are introduced in the superconductorlayer on the nano grooves. Each of the two-dimensional crystal defectsmay be continuous in a current flowing direction, it may be an array ofdiscontinuous two-dimensional crystal defects, or it may be irregularlydistributed on the substrate. The two-dimensional crystal defects may becrystal grain boundaries, dislocation arrays, amorphous bodies formedfrom elements constituting the superconductor layer, nonsuperconductorsor low critical temperature superconductors. Preferably, the nanogrooves may have a width of not more than 100 nm and a depth of not morethan 100 nm and the average center to center distance of the adjacentnano grooves in a direction perpendicular to a current flowing directionmay be not more than 500 nm. The substrate may be a substrate of anoxide having a perovskite type crystal structure, a rock-salt typecrystal structure, a spinel type crystal structure, an yttriumstabilized zirconia type structure, a fluorite type crystal structure, arare earth C type crystal structure, a pyrochlore type crystalstructure, and the like. Alternatively, the substrate may be an oxidesubstrate, a nitride substrate, a semiconductor substrate, anickel-based alloy substrate, a copper-based alloy substrate or aniron-based alloy substrate on the surface of which a buffer layer madeof the above-cited oxide or a boride is formed. The superconductor layermay be formed from a superconducting material selected from the groupconsisting of copper oxide-based high temperature superconductingmaterials having a chemical formula of LnBa₂Cu₃O_(7+x) where, Ln is oneor more elements selected from the group consisting of Y element andrare earth elements and −0.5<x<0.2; copper oxide-based high temperaturesuperconducting materials having a chemical formula of(Bi_(1−x)Pb_(x))₂Sr₂Ca_(n−1)Cu_(n)O_(2n+4+y), where 0<x<0.4, −0.5<y<0.5and n=1, 2 or 3; and superconducting materials which contain MgB₂ as amain component. Also, the superconductor layer may be formed from aplurality of layers and nano grooves may be formed in each of theplurality of layers except a top layer.

A superconducting film of the second embodiment of the invention has asubstrate and a superconductor layer formed on the substrate, in whichnano grooves are formed parallel to a current flowing direction on asubstrate surface on which the superconductor layer is formed, defectinducing parts are formed on the nano grooves, and the two-dimensionalcrystal defects are introduced in the superconductor layer on the defectinducing parts. Each of the two-dimensional crystal defects may becontinuous in a current flowing direction, it may be an array ofdiscontinuous two-dimensional crystal defects, or it may be irregularlydistributed on the substrate. The two-dimensional crystal defects may becrystal grain boundaries, dislocation arrays, amorphous bodies formedfrom elements constituting the superconductor layer, nonsuperconductorsor low critical temperature superconductors. Preferably, the nanogrooves may have a width of not more than 100 nm and a depth of not morethan 100 nm and the center to center distance of the adjacent nanogrooves in a direction perpendicular to a current flowing direction maybe not more than 500 nm. The substrate may be a substrate of an oxidehaving a perovskite type crystal structure, a rock-salt type crystalstructure, a spinel type crystal structure, an yttrium stabilizedzirconia type structure, a fluorite type crystal structure, a rare earthC type crystal structure, a pyrochlore type crystal structure.Alternatively, the substrate may be an oxide substrate, a nitridesubstrate, a semiconductor substrate, a nickel-based alloy substrate, acopper-based alloy substrate or an iron-based alloy substrate on thesurface of which a buffer layer made of the above-cited oxide or aboride is formed. The superconductor layer may be formed from asuperconducting material selected from the group consisting of copperoxide-based high temperature superconducting materials having a chemicalformula of LnBa₂Cu₃O_(7+x), where Ln is one or more elements selectedfrom the group consisting of Y element and rare earth elements and−0.5<x<0.2; copper oxide-based high temperature superconductingmaterials having a chemical formula of(Bi_(1−x)Pb_(x))₂Sr₂Ca_(n−1)Cu_(n)O_(2n+4+y), where 0<x<0.4, −0.5<y<0.5and n=1, 2 or 3; and superconducting materials which contain MgB₂ as amain component. The defect inducing parts may be formed from a metal, anintermetallic compound, a nitride or an oxide. Also, the superconductorlayer may be formed from a plurality of layers and nano grooves may beformed in each of the plurality of layers except a top layer.

A superconducting film of the third embodiment of the invention has asubstrate and a superconductor layer formed on the substrate, in whichrows of nano holes are formed parallel to a current flowing direction ona substrate surface on which the superconductor layer is formed and rowsof one-dimensional crystal defects are introduced in the superconductorlayer on the nano holes. Each of the rows of one-dimensional crystaldefects may be a row of one-dimensional crystal defects which iscontinuous in a current flowing direction, it may be an array ofdiscontinuous row of one-dimensional crystal defects, or it may beirregularly distributed on the substrate. The one-dimensional crystaldefects may be crystal grain boundaries, dislocation arrays, amorphousbodies formed from elements constituting the superconductior layer,nonsuperconductors or low critical temperature superconductors.Preferably, the nano holes may have a diameter of not more than 100 nmand the center to center distance of the adjacent nano holes in adirection perpendicular to a current flowing direction may be not morethan 500 nm. The substrate may be a substrate of an oxide having aperovskite type crystal structure, a rock-salt type crystal structure, aspinel type crystal structure, an yttrium stabilized zirconia typestructure, a fluorite type crystal structure, a rare earth C typecrystal structure, a pyrochlore type crystal structure. Alternatively,the substrate may be an oxide substrate, a nitride substrate, asemiconductor substrate, a nickel-based alloy substrate, a copper-basedalloy substrate or an iron-based alloy substrate on the surface of whicha buffer layer made of the above-cited oxide or a boride is formed. Thesuperconductor layer may be formed from a superconducting materialselected from the group consisting of copper oxide-based hightemperature superconducting materials having a chemical formula ofLnBa₂Cu₃O_(7+x), where Ln is one or more elements selected from thegroup consisting of Y element and rare earth elements and −0.5<x<0.2;copper oxide-based high temperature superconducting materials having achemical formula of (Bi_(1−x)Pb_(x))₂Sr₂Ca_(n−1)Cu_(n)O_(2n+4+y), where0<x<0.4, −0.5<y<0.5 and n=1, 2 or 3; and superconducting materials whichcontain MgB₂ as a main component. Also, the superconductor layer may beformed from a plurality of layers and nano holes may be formed in eachof the plurality of layers except a top layer.

A superconducting film of the fourth embodiment of the invention has asubstrate and a superconductor layer formed on the substrate, in whichrows of nano holes are formed parallel to a current flowing direction ona substrate surface on which the superconductor layer is formed, defectinducing parts are formed on the nano holes, and rows of one-dimensionalcrystal defects are introduced in the superconductor layer on the defectinducing parts. Each of the rows of one-dimensional crystal defects maybe a row of one-dimensional crystal defects which is continuous in acurrent flowing direction, it may be an array of discontinuous row ofone-dimensional crystal defects, or it may be irregularly distributed onthe substrate. The one-dimensional crystal defects may be crystal grainboundaries, dislocation arrays, amorphous bodies formed from elementsconstituting the superconductor layer, nonsuperconductors or lowcritical temperature superconductors. Preferably, the nano holes mayhave a diameter of not more than 100 nm and the average center to centerdistance of the adjacent nano holes in a direction perpendicular to acurrent flowing direction may be not more than 500 nm. The substrate maybe a substrate of an oxide having a perovskite type crystal structure, arock-salt type crystal structure, a spinel type crystal structure, anyttrium stabilized zirconia type structure, a fluorite type crystalstructure, a rare earth C type crystal structure, a pyrochlore typecrystal structure. Alternatively the substrate may be an oxidesubstrate, a nitride substrate, a semiconductor substrate, anickel-based alloy substrate, a copper-based alloy substrate or aniron-based alloy substrate on the surface of which a buffer layer madeof the above-cited oxide or a boride is formed. The superconductor layermay be formed from a superconducting material selected from the groupconsisting of copper oxide-based high temperature superconductingmaterials having a chemical formula of LnBa₂Cu₃O_(7+x), where Ln is oneor more elements selected from the group consisting of Y element andrare earth elements and −0.5<x<0.2; copper oxide-based high temperaturesuperconducting materials having a chemical formula of(Bi_(1−x)Pb_(x))₂Sr₂Ca_(n−1)Cu_(n)O_(2n+4+y), where 0<x<0.4, −0.5<y<0.5and n=1, 2 or 3; and superconducting materials which contain MgB₂ as amain component. The defect inducing parts may be formed from a metal, anintermetallic compound, a nitride or an oxide. Also, the superconductorlayer may be formed from a plurality of layers and nano holes may beformed in each of the plurality of layers except a top layer.

Superconducting films of the first and second embodiments of theinvention can be manufactured by a method comprising the steps offorming nano grooves on a substrate, optionally forming defect inducingparts on the nano grooves, and growing a superconductor layer on thesubstrate. The step of forming nano grooves may be performed bymechanical polishing, etching, nano imprint, AFM in processing mode ornano lithography. Preferably, the nano grooves may be formed in such amanner that the nano grooves have a width of not more than 100 nm and adepth of not more than 100 nm and the average center to center distanceof the adjacent nano grooves in a direction perpendicular to a currentflowing direction is not more than 500 nm. On the other hand, the stepof forming a superconductor layer may be performed by PLD, evaporation,sputtering, CVD, MBE or MOD process. Furthermore, the step of formingdefect inducing parts may be performed by PLD, evaporation, sputtering,CVD or MBE process.

Superconducting films of the third and fourth embodiments of theinvention can be manufactured by a method comprising the steps offorming rows of nano holes on a substrate, optionally forming defectinducing parts on the nano holes, and growing a superconductor layer onthe substrate. The step of forming rows of nano holes may be performedby mechanical polishing, etching, nano imprint, atomic force microscopy(AFM) in processing mode or nano lithography. Preferably, the nano holesmay be formed in such a manner that the nano holes have a diameter ofnot more than 100 nm and the average center to center distance of theadjacent rows of nano holes in a direction perpendicular to a currentflowing direction is not more than 500 nm. On the other hand, the stepof forming a superconductor layer may be performed by pulsed laserdeposition (PLD), evaporation, sputtering, chemical vapor deposition(CVD), molecular beam epitaxy (MBE) or metal-organic deposition (MOD)process. Furthermore, the step of forming defect inducing parts may beperformed by PLD, evaporation, sputtering, CVD or MBE process.

According to the present invention configured as described above, strongpinning centers having excellent pinning efficiency can be introducedinto a superconducting film, and a superconducting film having very highJc can be manufactured at low cost. Since the pinning centers introducedinto a superconducting film of the invention are aligned in a currentflowing direction, these pinning centers will not impede a currentflowing path. Therefore, a superconducting film of the invention isuseful in applications which require flowing large currents, such ascables, magnets, shields, current limiters, microwave devices, andsemifinished products of these articles.

The above and other objects, effects, features and advantages of thepresent invention will become more apparent from the followingdescription of the embodiments thereof taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view, in perspective, of a superconducting film ofthe first embodiment of the invention;

FIG. 2 is a sectional view, in perspective, of a superconducting film ofthe second embodiment of the invention;

FIG. 3 is a sectional view, in perspective, of a superconducting film ofthe third embodiment of the invention;

FIG. 4 is a sectional view, in perspective, of a superconducting film ofthe fourth embodiment of the invention;

FIG. 5A is a schematic top view of an example of the arrangement of nanogrooves in a superconducting film of the invention in which the nanogrooves are continuous;

FIG. 5B is a schematic top view of an example of the arrangement of nanogrooves in a superconducting film of the invention in which the nanogrooves are discontinuous;

FIG. 5C is a schematic top view of an example of the arrangement of nanogrooves in a superconducting film of the invention in which the nanogrooves are irregularly arranged;

FIG. 6A is a schematic top view of an example of the arrangement of nanoholes in a superconducting film of the invention in which rows of thenano holes are continuous;

FIG. 6B is a schematic top view of an example of the arrangement of nanoholes in a superconducting film of the invention in which rows of thenano holes are discontinuous; and

FIG. 6C is a schematic top view of an example of the arrangement of nanoholes in a superconducting film of the invention in which rows of thenano holes are irregularly arranged.

BEST MODE FOR CARRYING OUT THE INVENTION

A superconducting film of the first embodiment of the invention is shownin FIG. 1. The superconducting film of FIG. 1 has a substrate 1 and asuperconductor layer 3 formed on the substrate 1. Nano grooves 2 areformed parallel to a current flowing direction on a substrate surfacewhere the superconductor layer 3 is formed, and two-dimensional crystaldefects 4 are introduced in the superconductor layer on the nano grooves2. The two-dimensional crystal defects 4 function as two-dimensionalpinning centers.

As the substrate 1 may be used a substrate of an oxide having perovskitetype crystals, such as SrTiO₃ and LaAlO₃; rock-salt type crystals, suchas MgO and NiO; spinel type crystals, such as MgAl₂O₄; yttriumstabilized zirconia; fluorite type crystals, such as CeO₂; rare earth Ctype crystals; and pyrochlore type crystals. Alternatively, substrateswith a buffer layer may be used as the substrate 1: wherein thesubstrates on which the buffer layer is formed may be a substrate of theabove oxides, a nitride substrate, a semiconductor substrate, asubstrate of pure nickel, a nickel-based alloy substrate of Ni—Cr, Ni—Wand the like, a substrate of pure copper, a copper-based alloy substrateof Cu—Ni and the like, or an iron-based alloy substrate of Fe—Si,stainless steel and the like; and wherein the buffer layer, which isformed on the surface of the substrate, may be made of the above-citedoxide or a boride (MgB₂ etc.). By using such substrates, it becomespossible to form a superconductor layer 3 comprising a superconductorwhich is c-axis oriented on the surface of the substrate 1.

The nano grooves 2 are a plurality of grooves formed in the substrate 1,and have a width of not more than 100 nm, and preferably 5 nm to 50 nm,and a depth of not more than 100 nm, and preferably 5 nm to 50 nm. It isdesirable that the width of the nano grooves be larger than the diameterof quantized flux lines (twice the coherent length ξ of thesuperconducting material), depending on the superconducting materialconstituting the superconductor layer 3. It is desirable that theaverage center to center distance of the plurality of nano grooves in adirection perpendicular to a current flowing direction be smaller thanthe lattice constant of quantized flux line lattice a_(f)(=1.07×(f₀/B)^(1/2), B denotes a magnetic field applied to thesuperconductor layer 3). The average center to center distance of thenano grooves is usually not more than 500 nm, preferably 15 nm to 300nm, and more preferably 20 nm to 200 nm, depending on applied magneticfield B. When the nano grooves have widths, depths and average center tocenter distances in the above-described ranges, the quantized flux linesin the superconductor layer can be efficiently pinned.

Each of the nano grooves 2 may be continuous in the current flowingdirection of the superconducting film (see FIG. 5A) or may be an arrayof discontinuous grooves (see FIG. 5B). It is desirable that thedistance between two nano grooves in a discontinuous part in a currentflowing direction be smaller than the lattice constant of quantized fluxline lattice a_(f) in the superconductor layer 3. This distance isusually not more than 500 nm, preferably 15 nm to 300 nm, and morepreferably 20 nm to 200 nm, depending on a magnetic field B applied tothe superconductor layer 3. Alternatively, the plurality of nano grooves2 may be irregularly arranged on the substrate 1, provided that themajor axis of the nano grooves is parallel to a current flowingdirection (see FIG. 5C). Also in this case, it is desirable that theaverage center to center distance of the nano grooves be within theabove-described ranges. Furthermore, when nano grooves 2 arediscontinuous or irregularly arranged, it is desirable thatdiscontinuous parts be not align in a direction perpendicular to acurrent flowing direction. This is because if the discontinuous partsare aligned in this direction, the pinning effect of the quantized fluxlines decreases in these parts.

The materials which constitute the superconductor layer 3 may be copperoxide-based high temperature superconducting materials having thechemical formula of LnBa₂Cu₃O_(7+x) where, Ln is one or more elementsselected from the group consisting of Y element and rare earth elements(elements with atomic number 57 to 71) and −0.5<x<0.2; copperoxide-based high temperature superconducting materials having thechemical formula of (Bi_(1−x)Pb_(x))₂Sr₂Ca_(n−1)Cu_(n)O_(4+y), where0<x<0.4, −0.5<y<0.5 and n=1, 2 or 3; or superconducting materials whichcontain MgB₂ as a main component. Superconducting materials whichcontain MgB₂ as a main component in the present invention mean MgB₂which may contain carbon, oxygen or SiC etc. as impurities. Thesematerials are deposited onto a surface of substrate 1 in a c-axisoriented state (the c-axis of these materials is parallel to the normalline of the substrate surface) to form the superconductor layer 3 havinga superconducting plane parallel to the substrate plane. Thesuperconductor layer 3 usually has a film thickness in a range of 0.1 μmto 10 μm and preferably in a range of 0.1 μm to 5 μm.

The nano grooves 2 on the substrate 1 can be formed by using mechanicalpolishing (nano scratching), etching, nano imprint, AFM in processingmode or nano lithography. A preferred method includes nano scratching,nano imprint and AFM in processing mode. For example, nano scratchingand nano imprint can be performed by polishing with abrasive grains ofdiamond etc. in a current flowing direction; or pressing a jig providedwith microprotrusions having a desired shape and intervals against thesubstrate 1 and then moving the jig in a current flowing direction.Alternatively, the nano grooves 2 can be formed by continuouslyprocessing the substrate with an AFM in which a high voltage is appliedto a probe.

The superconductor layer 3 can be formed by using PLD, evaporation,sputtering, CVD, MBE or MOD process. When the superconductor layer 3 isformed on a surface of the substrate 1 provided with the nano grooves 2,a film which has grown on a flat portion and a film formed on the nanogrooves 2 have different crystal orientations. Therefore, it followsthat, in an area where the two meets, dislocations and/or crystal grainboundaries are formed. Furthermore, on the nano grooves, there is apossibility that amorphous bodies are formed, or alternatively, crystalshaving many defects are formed due to the occurrence of compositionalvariation. As a result, a layer of a nonsuperconductor or a layer of alow critical temperature superconductor is formed on the nano grooves 2.In this specification, the dislocations, crystal grain boundaries,amorphous bodies, nonsuperconductors and low critical temperaturesuperconductors in the superconductor layer 3 are collectively called“crystal defects.” These crystal defects will not disappear with thegrowth of a film and form two-dimensional crystal defects 4 continuingfrom the nano grooves 2 on the substrate to the surface of thesuperconductor layer 3. Although it is not always necessary for thetwo-dimensional crystal defects 4 to be perpendicular to the substrateplane, it is desirable that the two-dimensional crystal defects 4 bepresent at an angle close to an angle perpendicular to the substrateplane. These two-dimensional crystal defects 4, which have nosuperconducting properties or have poor superconducting properties,function as two-dimensional pinning centers.

According to this arrangement, the two-dimensional crystal defects 4 arearranged parallel to a current flowing direction and, therefore, they donot impede the flow of a current. When a magnetic field is applied tothe superconductor layer 3 perpendicularly thereto, quantized flux linestend to move toward the two-dimensional crystal defects 4. This isbecause the Lorentz force acting on the quantized flux lines works in adirection parallel to the substrate plane and orthogonal to the flow ofa current. However, two-dimensional pinning centers (two-dimensionalcrystal defects 4) can work to pin all quantized flux lines, since thetwo-dimensional crystal defects 4 of the invention pin even quantizedflux lines which tend to move by overcoming the interaction in flux linelattices. Compared to dot-like pinning centers such as oxygen deficiencyand impurities or one-dimensional pinning centers such as dislocationsand columnar defects which are distributed randomly, the two-dimensionalpinning centers of the invention, which are regularly arranged in acurrent flowing direction, have very high pinning efficiency.

The above-described effect is attributed to the issue of dimensionalitythat pinning with two-dimensional pinning centers is superior in pinningquantized flux lines which have a string shape. Since thetwo-dimensional pinning center of the invention (two-dimensional crystaldefects 4) can pin a larger number of quantized flux lines with asmaller number of pinning centers, and thereby Jc in a magnetic field isimproved. Furthermore, the two-dimensional crystal defects 4 work veryeffectively without interrupting a current path contrary to generalgrain boundaries which occur randomly in a superconductor layer, sincethey are parallel to a current flowing direction. Although thetwo-dimensional crystal defects 4 desirably extend continuously in acurrent flowing direction, it is not always necessary that they becontinuous, and they may be discontinuous as described above. The sameeffect is obtained even when discontinuous two-dimensional defects areirregularly distributed on a substrate, provided that the major axisdirection of discontinuous two-dimensional defects is parallel to acurrent flowing direction.

The thickness of the two-dimensional crystal defects 4 can be controlledby adjusting the width of the nano grooves 2. Since the two-dimensionalcrystal defects 4 are dislocations, grain boundaries, amorphous bodies,nonsuperconductors or superconductors having a low critical temperature,they have the pinning interaction of quantized flux lines. Furthermore,the magnitude of the pinning force can be controlled, by controlling thesize of the two-dimensional crystal defects 4 to adjust the depth of thepinning potential and the potential steepness. The pinning energy ofquantized flux lines per unit length is expressed by(½ μ₀)Bc²×πξ²where μ₀ is magnetic permeability in a vacuum, Bc is the thermodynamiccritical magnetic field of the material for the superconductor layer 3,and ξ is the coherent length. The length of ξ is temperature dependent.Therefore, when the size of an optimum pinning center (two-dimensionalcrystal defect 4) varies with the working temperature of asuperconducting film, an optimum value of the pinning force can beselected by changing the width and average center to center spacing ofthe nano grooves.

As an alternative to this first embodiment, a superconducting materialmay be used as a buffer layer. That is, after a thin buffer layer of asuperconducting material is first formed on a substrate 1, nano grooves2 are formed by the same method as described above and a superconductorlayer 3 may be formed thereafter. Also in this case, two-dimensionalcrystal defects 4 are formed in the superconductor layer 3 on the nanogrooves 2. It is preferred that usable superconducting materials be thesame oxide as the material for the superconductor layer 3 or a boride.For example, when the superconductor layer 3 is formed fromLnBa₂Cu₃O_(7+x), a buffer layer may be formed from the sameLnBa₂Cu₃O_(7+x) or may be formed from a material in which only Ln isreplaced. A buffer layer in an area where no nano groove is formed hasthe effect of facilitating the epitaxial growth of the superconductorlayer 3 in this area, since this buffer layer is a superconducting filmof the same type as the superconductor layer 3.

As another alternative to this first embodiment, a superconductor layer3 may be formed from a plurality of layers and nano grooves may beformed in each of the plurality of layers except a top layer. Thisalternative embodiment is suitable for introducing two-dimensionalcrystal defects 4 at a predetermined density in a case where thesuperconductor layer 3 is thick and the distribution of two-dimensionalcrystal defects decreases as the formation of the superconductor layer 3progresses.

A superconducting film of the second embodiment of the invention isshown in FIG. 2. The superconducting film of FIG. 2 has a substrate 1 inwhich nano grooves 2 are formed parallel to a current flowing directionon the surface where a superconductor layer 3 is formed, defect inducingparts 5 formed on the nano grooves 2, and the superconductor layer 3formed on the substrate 1 and the defect inducing parts 5, andtwo-dimensional crystal defects 4 are introduced in the superconductorlayer 3 on the defect inducing parts 5. The two-dimensional crystaldefects 4 function as two-dimensional pinning centers. The substrate 1,the nano grooves 2 and the superconductor layer 3 are the same as in thefirst embodiment.

The defect inducing parts 5 are formed from plate-like crystals orseries of island-like crystals. The nano grooves 2 function aspreferential nucleation sites, and thereby, the defect inducing parts 5are formed on the nano grooves 2. Usable materials include, for example,metals such as Ag and Pt (it is desirable that the metals have a highmelting point); intermetallic compounds such as AgY and Pt₃Y; nitridessuch as GdN and YN; and oxides such as Y₂O₃ and CeO₂. Although in thepresent invention, it is preferred that the defect inducing parts 5 beformed from a material different from that of the substrate 1, thedefect inducing parts 5 may be formed from a material which is of thesame kind as the substrate 1 but has a different crystal orientation.The defect inducing parts 5 can be formed by depositing theabove-described materials on the substrate 1 by a method selected fromPLD, evaporation, sputtering CVD and MBE. In this case, such materialsnucleate and grow on the nano grooves 2, since the nano grooves 2 arepreferential nucleation sites compared to the flat substrate 1. Byadjusting the material supply time, film forming time and film formingtemperature, plate-like crystals or series of island-like crystals of anappropriate size can be formed on the nano grooves 2. Whether crystalsgrow in a plate form or in an island form is adjusted by appropriatelyselecting the wettability of the substrate 1 with the above-describedmaterials.

Unlike nano dots which are formed randomly on a substrate the defectinducing parts 5 are regularly arranged so that their shape becomesparallel to a current flowing direction, and in this respect theinvention is greatly different from prior art. Since the smoothness ofthe surf aces of the defect inducing parts 5, and/or the depositionrate, crystal orientation, etc. of a superconducting material on thesurfaces of the defect inducing parts 5 are different from those of thesubstrate 1, the two-dimensional crystal defects 4 are formed in thesuperconductor layer 3 formed on the defect inducing parts 5. Thesetwo-dimensional crystal defects 4 function as two-dimensional pinningcenters in the same manner as in the first embodiment, they give anexcellent pinning efficiency.

Also in this constitution, even quantized flux lines which tend to moveby overcoming the interaction between flux line lattices can be pinned,since the two-dimensional crystal defects 4 are arranged parallel to acurrent flowing direction. Therefore, two-dimensional pinning centerscan be obtained having a very high pinning efficiency.

The thickness of the two-dimensional crystal defects 4 can be controlledby adjusting the width of the defect inducing parts 5 (i.e., the widthof the nano grooves 2). Furthermore, the magnitude of the pinning forcecan be controlled, by controlling the size of the two-dimensionalcrystal defects 4 to adjust the depth of the pinning potential and thepotential steepness. When the size of an optimum pinning center(two-dimensional crystal defect 4) varies with the working temperatureof a superconducting film, an optimum value of the pinning force can beselected by changing the width and average center to center gap of thenano grooves.

As an alternative to this second embodiment, in the same manner as inthe first embodiment, a superconducting material may be used as a bufferlayer. Also in this case, two-dimensional crystal defects 4 are formedon defect inducing parts 5. Usable superconducting materials are thesame as in the first embodiment, and a buffer layer in an area where nodefect inducing part 5 is formed has the effect of facilitating theepitaxial growth of the superconductor layer 3 in this portion, sincethis buffer layer is a superconducting film of the same type as thesuperconductor layer 3.

As another alternative to this second embodiment, a superconductor layer3 may be formed from a plurality of layers and nano grooves may beformed in each of the plurality of layers except a top layer. Thisalternative embodiment is suitable for introducing two-dimensionalcrystal defects 4 of a predetermined density in a case where thesuperconductor layer 3 is thick and the distribution of two-dimensionalcrystal defects decreases as the formation of the superconductor layer 3progresses.

A superconducting film of the third embodiment of the invention is shownin FIG. 3. The superconducting film of FIG. 3 has a substrate 1 and asuperconductor layer 3 formed on the substrate 1. Rows of nano holes 6are formed parallel to a current flowing direction on a substratesurface where the superconductor layer 3 is formed, and one-dimensionalcrystal defects 7 are introduced in the superconductor layer on the nanoholes 6. The one-dimensional crystal defects 7 function as pinningcenters. The substrate 1 and the superconductor layer 3 are the same asin the first embodiment.

The nano holes 6 are a plurality of non-through holes which are formedon the substrate and are formed on the substrate 1 to form rows parallelto a current flowing direction. “Rows of the nano holes 6” means thatthe spacing between the adjacent nano holes 6 in a direction parallel toa current flowing direction is smaller than the lattice constant ofquantized flux line lattice a_(f) in the superconductor layer 3. Thespacing between the adjacent nano holes 6 in a direction parallel to acurrent flowing direction is usually not more than 250 nm, andpreferably in the range of 20 nm to 150 nm, depending on a magneticfield B applied to the superconductor layer 3. Rows of theone-dimensional crystal defects 7 formed by the rows of nano holes 6having such a spacing are dense relative to quantized flux lines andhave the same effect as two-dimensional pinning centers. That is,quantized flux lines can be effectively prevented from moving throughthe rows of the one-dimensional crystal defects 7.

It is desirable that the diameter of the nano holes 6 be larger than thediameter of the quantized flux lines constituting the superconductorlayer 3 (twice the coherent length ξ of a superconducting material). Itis desirable that the nano holes have a diameter of usually not morethan 100 nm, and preferably in the range of 5 nm to 50 nm, depending onthe material for the superconductor layer 3. The average is center tocenter distance between the rows of the nano holes 6 is usually not morethan 500 nm, preferably 15 to 300 nm, and more preferably 20 nm to 200nm, depending on an applied magnetic field B. When the rows of the nanoholes 6 have widths, depths and center to center distances in theabove-described ranges, the quantized flux lines in the superconductorlayer 3 can be pinned efficiently.

Each of the rows of the nano holes 6 may be continuous in the currentflowing direction of the superconducting film (see FIG. 6A) or may be anarray of discontinuous rows (see FIG. 6B). It is desirable that thedistance between two rows of nano holes in a discontinuous part in acurrent flowing direction be smaller than the lattice constant ofquantized flux line lattice a_(f) in the superconductor layer 3. Thisdistance is usually not more than 500 nm, preferably 15 nm to 300 nm,and more preferably 20 nm to 200 nm, depending on a magnetic field Bapplied to the superconductor layer 3. Alternatively, a plurality ofrows of nano holes 6 may be irregularly arranged on the substrate 1,provided that the major axis of the rows of nano holes be parallel to acurrent flowing direction (see FIG. 6C). Also in this case, it isdesirable that the distance between adjacent rows of nano holes 6 in acurrent flowing direction and the average center to center distance in adirection parallel to a current flowing direction be within theabove-described ranges. In the cases of FIG. 6B and FIG. 6C, it isdesirable that discontinuous parts be not aligned in a directionperpendicular to a current flowing direction. This is because ifdiscontinuous parts are aligned in this direction, the pinning effect ofthe quantized flux lines decreases in these parts. In the cases of FIG.6A to FIG. 6C, in two rows of nano holes which are adjacent in adirection orthogonal to a current flowing direction, the nano holes maybe arranged in a “mutually nested” state (i.e. positions of nano holesof one row correspond to positions of spacing of another row and viceversa, the state of FIG. 6A and FIG. 6B), or the nano holes may bealigned in a direction orthogonal to a current flowing direction.

The nano holes 6 on the substrate 1 can be formed by using mechanicalpolishing (nano scratching), etching, nano imprint, AFM in processingmode or nano lithography. A preferred method includes nano imprint andAFM in processing mode. For example, nano imprint can be performed bypressing a jig provided with microprotrusions having a desired shape andintervals against the substrate 1. Alternatively, the nano holes 6 canbe formed by intermittently processing the substrate using an AFM inwhich a high voltage is applied to a probe.

When the superconducting layer 3 is formed on the surface of thesubstrate where the nano holes 6 are provided, one-dimensional “crystaldefects” are formed on the nano holes 6 in the same manner as in thecase where the nano grooves 2 are provided. The crystal defects 7 on thenano holes 6 are the same as the “crystal defects” of the firstembodiment, with the exception that the crystal defects 7 on the nanoholes 6 are one-dimensional in shape, rather than two-dimensional. Thecrystal defects on the nano holes 6 will not disappear with the growthof a film and form one-dimensional crystal defects 7 continuing from thenano holes 6 on the substrate to the surface of the superconductor layer3. These one-dimensional crystal defects 7, which have nosuperconducting properties or have poor superconducting properties,become one-dimensional pinning centers. Although it is not alwaysnecessary for the one-dimensional crystal defects 7 to be perpendicularto the substrate plane, it is desirable that the one-dimensional crystaldefects 7 be present at an angle close to an angle perpendicular to thesubstrate plane.

According to the arrangement of this embodiment, the rows ofone-dimensional crystal defects 7 will not interrupt a current, sincethey are arranged parallel to a current flowing direction. When amagnetic field is applied to the superconductor layer 3 perpendicularlythereto, the quantized flux lines tend to move toward the sides of therows of one-dimensional crystal defects 7. This is because the Lorentzforce is in a direction parallel to the substrate surface and orthogonalto the flow of a current. However, if the rows of one-dimensionalcrystal defects 7 of the invention are dense, the rows ofone-dimensional crystal defects 7 can pin even quantized flux lineswhich tend to move by overcoming the interaction between flux linelattices, and hence the rows of one-dimensional crystal defects 7 canwork for the pinning of all quantized flux lines. The structure of thepinning centers of the invention has very high pinning efficiency.

The diameter of the one-dimensional crystal defects 7 can be controlledby adjusting the diameter of the nano holes 6. Since the one-dimensionalcrystal defects 7 are dislocations, grain boundaries, amorphous bodies,nonsuperconductors or superconductors having a low critical temperature,they have the pinning interaction of quantized flux lines. Furthermore,the magnitude of the pinning force can be controlled, by controlling thesize of the nano holes 6 to adjust the depth of the pinning potentialand the potential steepness. When the size of an optimum pinning center(one-dimensional crystal defect 7) varies with the working temperatureof a superconducting film, an optimum value of the pinning force can beselected by changing the diameter of nano holes, the spacing between thenano holes in a current flowing direction, and the average center tocenter spacing of the rows of nano holes in a direction orthogonal to acurrent flowing direction.

As an alternative to this third embodiment, a superconducting materialmay be used as a buffer layer. That is, after a buffer layer of asuperconducting material is first formed thin on a substrate 1, nanoholes 6 are formed by the same method as described above and asuperconductor layer 3 may be formed thereafter. Also in this case,one-dimensional crystal defects 7 are formed in the superconductor layer3 on the nano holes 6. It is preferred that usable superconductingmaterials be the same oxide as the material for the superconductor layer3 or a boride. For example, when the superconductor layer 3 is formedfrom LnBa₂Cu₃O_(7+x), a buffer layer may be formed from the sameLnBa₂Cu₃O_(7+x) or may be formed from a material in which only Ln isreplaced. A buffer layer in an area where no nano hole is formed has theeffect of facilitating the epitaxial growth of the superconductor layer3 in this portion, since this buffer layer is a superconducting film ofthe same type as the superconductor layer 3.

As another alternative to this third embodiment, a superconductor layer3 may be formed from a plurality of layers and nano holes may be formedin each of the plurality of layers except a top layer. This alternativeembodiment is suitable for introducing one-dimensional crystal defects 7at a predetermined density in a case where the superconductor layer 3 isthick and the distribution of one-dimensional crystal defects decreasesas the formation of the superconductor layer 3 progresses.

A superconducting film of the fourth embodiment of the invention isshown in FIG. 4. The superconducting film of FIG. 4 has a substrate 1 inwhich rows of nano holes 6 are formed parallel to a current flowingdirection on the surface where a superconductor layer 3 is formed,defect inducing parts 8 formed on the rows of the nano holes 6, and thesuperconductor layer 3 formed on the substrate 1 and the defect inducingparts 8, and the one-dimentional crystal defect 7 is introduced in thesuperconductor layer on the defect inducing parts 8. The one-dimensionalcrystal defects 7 function as pinning centers. The substrate 1, the nanoholes 6 and the superconductor layer 3 are the same as in the thirdembodiment.

The defect inducing parts 8 are the same as the defect inducing parts 5of the second embodiment, with the exception that the shape isisland-like crystals. Usable materials include, for example, metals suchas Ag and Pt (it is desirable that the metals have a high meltingpoint); intermetallic compounds such as AgY and Pt₃Y, nitrides such asGdN and YN; and oxides such as RE₂O₃ (RE: rare earth element) and CeO₂.Although in the present invention, it is preferred that the defectinducing parts 8 be formed from a material different from that of thesubstrate 1, the defect inducing parts 8 may be formed from a materialwhich is of the same kind as the substrate 1 but has a different crystalorientation. The defect inducing parts 8 can be formed by depositing theabove-described materials on the substrate 1 by a method selected fromPLD, evaporation, sputtering CVD and MBE. In this case, the nano holes 6are preferential nucleation sites compared to the flat substrate 1 and,therefore, the above-described materials nucleate and grow on the nanoholes 6. By adjusting the material supply amount, film forming time andfilm forming temperature, it is possible to form the defect inducingparts 8 consisting of island-like crystals of an appropriate size on thenano holes 6.

Unlike nano dots which are formed randomly on a substrate, the rows ofdefect inducing parts 8 are regularly arranged so as to be parallel to acurrent flowing direction, and in this respect the invention is greatlydifferent from prior art. Since the smoothness of the surfaces of thedefect inducing parts 8, and/or the deposition rate, crystalorientation, etc. of a superconducting material on the surfaces of thedefect inducing parts 8 are different from those of the substrate 1, theone-dimensional crystal defects 7 are formed in the superconductor layer3 formed on the defect inducing parts 8. Rows of these one-dimensionalcrystal defects 7 function as one-dimensional pinning centers in thesame manner as in the third embodiment. Because the rows ofone-dimensional crystal defects 7 are arranged parallel to a currentflowing direction, they will not interrupt a current. Therefore, therows of one-dimensional crystal defects 7 of this embodiment can provideone-dimensional pinning centers having an excellent pinning efficiency.

The diameter of the one-dimensional crystal defects 7 can be controlled,by adjusting the diameter of the defect inducing parts 8 (i.e., thewidth of the nano holes 6). Furthermore, the magnitude of the pinningforce can be controlled by controlling the size of the one-dimensionalcrystal defects 7 to adjust the depth of the pinning potential and thepotential steepness. When the size of an optimum pinning center(one-dimensional crystal defect 7) varies with the working temperatureof a superconducting film, an optimum value of the pinning force can beselected by changing the diameter of the nano holes 6 and the averagecenter to center spacing of the rows of nano holes 6.

As an alternative to this fourth embodiment, a superconducting materialmay be used as a buffer layer in the same manner as in the thirdembodiment. Also in this case, one-dimensional crystal defects 7 areformed on the defect inducing parts 8. It is preferred that usablesuperconducting materials be the same oxide as the material for thesuperconductor layer 3 or a boride. For example, when the superconductorlayer 3 is formed from LnBa₂Cu₃O_(7+x), a buffer layer may be formedfrom the same LnBa₂Cu₃O_(7+x) or may be formed from a material in whichonly Ln is replaced. A buffer layer in an area where no defect inducingpart is formed has the effect of facilitating the epitaxial growth ofthe superconductor layer 3 in this portion, since this buffer layer is asuperconducting film of the same type as the superconductor layer 3.

As another alternative to this fourth embodiment, a superconductor layer3 may be formed from a plurality of layers and rows of nano holes 6 maybe formed in each of the plurality of layers except a top layer. Thisalternative embodiment is suitable for introducing one-dimensionalcrystal defects 7 of a prescribed density in a case where thesuperconductor layer 3 is thick and the distribution of one-dimensionalcrystal defects 7 decreases as the formation of the superconductor layer3 progresses.

EXAMPLE 1

First, nano grooves were formed on a single-crystal substrate. An SrTiO₃substrate 3 mm wide×10 mm long×0.5 mm thick, the (100) plane of whichwas mirror-like polished, was prepared as the single-crystal substrate.Subsequently, nano grooves were formed in a region of 60 μm×60 μm in thecentral part of this substrate surface by utilizing AFM in processingmode. With the width and depth of the nano grooves set at 30 nmrespectively and the length of the nano grooves at 60 μm, 330 nanogrooves were formed in the above-described region with equal spacingsuch that the nano grooves become parallel to the longitudinal directionof the substrate. The average spacing of the centers of the nano grooveswas 150 nm. With the substrate fixed on a heater provided within avacuum chamber, a thin film of YBa₂Cu₃O_(7−x) (YBCO) was formed on thesubstrate by the excimer pulsed laser deposition (PLD) method and asuperconducting film (I-1) was obtained. At this time, a flat SrTiO₃substrate 3 mm wide×10 mm long×0.5 mm thick, in which no nano groove wasformed, was fixed beside the substrate on which nano grooves were formedand a superconducting film (C-1) was obtained. The superconducting film(I-1) is an example of the invention and the superconducting film (C-1)is a comparative example outside the scope of the invention. A target ofsintered YBCO of a stoichiometric composition was used in the above PLDmethod. The substrate temperature during film deposition was 780° C. Thepartial oxygen pressure was 200 mTorr, and a sufficient volume of oxygenwas introduced in the film cooling process. The film thickness of theobtained YBCO was 0.5 μm.

The crystal orientation of the two films (I-1) and (C-1) was evaluatedby X-ray diffraction and it was ascertained that both are highly c-axisoriented films. Furthermore, when the in-plane crystal orientation ofthe films was investigated by φ scan, it was found that both were highlyin-plane oriented. From these investigation, it could be ascertainedthat the two films are biaxially oriented to the same extent.

In order to investigate the superconducting properties of the obtainedfilms, a bridge pattern was formed on the YBCO films by lithography. Thewidth of the bridge was 40 μm and its length was 40 μm. At this time, inthe superconducting film (I-1), the bridge pattern was formed on theabove-described region 60 μm×60 μm where the nano grooves were formed.Consequently, when energized, a current flows parallel to the nanogrooves. For the patterned two samples, the electrical properties wereevaluated by the four terminal method. The critical temperature Tc ofthe samples was 90 K for the superconducting film (I-1) with nanogrooves and 91 K for the superconducting film (C-1) without a nanogroove, as determined from temperature variations in resistivity. Undera zero magnetic field, the critical current density Jc of thesuperconducting film (I-1) at 77 K was 5,000,000 A/cm², and that of thesuperconducting film (C-1) was 4,500,000 A/cm². Furthermore, under amagnetic field of 1 T (tesla) parallel to the c-axis (perpendicular tothe substrate plane), Jc of the superconducting film (I-1) at atemperature of 77 K was 1,100,000 A/cm², and that of the superconductingfilm (C-1) was 580,000 A/cm².

EXAMPLE 2

Nano grooves were formed on an SrTiO₃ substrate in the same manner as inExample 1. Next, the substrate on which nano grooves had been formed wasfixed on a heater within a vacuum heater for PLD, and defect inducingparts were formed on the nano grooves by the PLD method. Y₂O₃ was usedas the material for the defect inducing parts. A Y₂O₃ sintered compacttarget was ablated using 30 pulses of an excimer laser, and Y₂O₃ wasdeposited on the SrTiO₃ substrate. At this time, the substratetemperature was 700° C. and the partial oxygen pressure was 10⁻⁵ Torr(1.33×10⁻³ Pa). Under these conditions, Y₂O₃ deposited only on the nanogrooves to form the defect inducing parts. Then, the temperature waslowered to room temperature and in the same manner as in Example 1, anSrTiO₃ substrate without a nano groove was attached on the heater,beside the sample with nano grooves and defect inducing parts. Then, inthe same manner as in Example 1, YBCO films were formed by the PLDmethod using the sintered YBCO target, to give superconducting films(I-2) and (C-2). The superconducting film (I-2) is an example of theinvention having defect inducing parts, and the superconducting film(C-2) is a comparative example outside the scope of the invention. Thefilm deposition conditions were the same as in Example 1.

According to an X-ray diffraction analysis, the two samples equallyshowed a high c-axis orientation and a high in-plane orientation.Subsequently, in order to investigate the superconducting properties ofthe obtained films, a bridge pattern was formed on the YBCO films bylithography. The width of the bridge was 40 μm and its length was 40 μm.At this time, in the superconducting film (I-2), the bridge pattern wasformed on the above-described region 60 μm×60 μm where the nano grooveswere formed. The critical temperature Tc of the two patterned sampleswas measured 89.5 K for the superconducting film (I-2) and 90.5 K forthe superconducting film (C-2). Under a zero magnetic field, thecritical current density Jc at 77 K of the superconducting film (I-2)was 5,200,000 A/cm², and that of the superconducting film (C-2) was4,300,000 A/cm². Furthermore, under a magnetic field of 1 T parallel tothe c-axis, Jc of the superconducting film (I-2) at a temperature of 77K was 1,300,000 A/cm², and that of the superconducting film (C-2) was550,000 A/cm².

EXAMPLE 3

First, nano holes were formed on a single-crystal substrate. An SrTiO₃substrate 3 mm wide×10 mm long×0.5 mm thick, the (100) plane of whichwas mirror-like polished, was prepared as the single-crystal substrate.Rows of nano holes were formed in a region of 60 μm×60 μm in the centralpart of this substrate surface by use of electron beam lithography. Thewidth and depth of the nano holes were 40 nm and 20 nm, respectively.The rows of nano holes were arranged so as to be parallel to the lengthdirection of the substrate and so that the length of the rows became 60μm. The spacing between the nano holes in the longitudinal direction ofthe substrate was 100 nm, and 330 rows of nano holes were formed in theabove-described region 60 μm×60 μm with equal spacing. The averagespacing of the centers of the nano hole rows was 150 nm. With thissubstrate fixed on a heater provided within a vacuum chamber, a thinfilm of YBa₂Cu₃O_(7−x) (YBCO) was formed on the substrate by the excimerpulse laser deposition (PLD) method and a superconducting film (I-3) wasobtained. At this time, a flat SrTiO₃ substrate 3 mm wide×10 mm long×0.5mm thick, in which no nano hole was formed, was fixed beside thesubstrate on which nano holes were formed and a superconducting film(C-3) was obtained. The film deposition conditions were the same as inExample 1.

According to an X-ray diffraction analysis, the two samples equallyshowed a high c-axis orientation and a high in-plane orientation.Subsequently, in order to investigate the superconducting properties ofthe obtained films, abridge pattern was formed on the YBCO films bylithography. The width of the bridge was 40 μm and its length was 40 μm.At this time, in the superconducting film (I-3), the bridge pattern wasformed on the above-described region 60 μm×60 μm where the rows of nanoholes were formed. The critical temperature Tc of the two patternedsamples was measured 90.5 K for the superconducting film (I-3) and 91 Kfor the superconducting film (C-3). Under a zero magnetic field, thecritical current density Jc of the superconducting film (I-3) at 77 Kwas 5,100,000 A/cm², and that of the superconducting film (C-3) was4,000,000 A/cm². Furthermore, under a magnetic field of 1 T parallel tothe c-axis, Jc of the superconducting film (I-3) at a temperature of 77K was 1,000,000 A/cm², and that of the superconducting film (C-3) was450,000 A/cm².

EXAMPLE 4

Nano holes were formed on an SrTiO₃ substrate in the same manner as inExample 3. Next, the substrate on which nano holes had been formed wasfixed on a heater within a vacuum heater for PLD, and defect inducingparts were formed on the nano holes by the PLD method. Y₂O₃ was used asthe material for the defect inducing parts. A Y₂O₃ sintered compacttarget was ablated using 15 pulses of an excimer laser and Y₂O₃ wasdeposited on the SrTiO₃ substrate. At this time, the substratetemperature was 700° C. and the partial oxygen pressure was 10⁻⁵ Torr(1.33×10⁻³ Pa). Under these conditions, Y₂O₃ deposited only on the nanoholes to form the defect inducing parts. Then, the temperature waslowered to room temperature and in the same manner as in Example 3, aflat SrTiO₃ substrate without a nano hole was attached on the heater,beside the sample with defect inducing parts. Then, in the same manneras in Example 3, YBCO films were formed by the PLD method using thesintered YBCO target, and superconducting films (I-4) and (C-4) wereobtained. The superconducting film (I-4) is an example of the inventionhaving defect inducing parts and the superconducting film (C-4) is acomparative example outside the scope of the invention.

According to an X-ray diffraction analysis, the two samples equallyshowed a high c-axis orientation and a high in-plane orientation.Subsequently, in order to investigate the superconducting properties ofthe obtained films, abridge pattern was formed on the YBCO films bylithography. The width of the bridge was 40 μm and its length was 40 μm.At this time, in the superconducting film (I-4), the bridge pattern wasformed on the above-described region 60 μm×60 μm where the defectinducing parts (nano holes) were formed. The critical temperature Tc ofthe two patterned samples was measured 90 K for the superconducting film(I-4) and 90.5 K for the superconducting film (C-4). Under a zeromagnetic field, the critical current density Jc of the superconductingfilm (I-4) at 77 K was 4,800,000 A/cm², and that of the superconductingfilm (C-4) was 4,500,000 A/cm². Furthermore, under a magnetic field of 1T parallel to the c-axis, Jc of the superconducting film (I-4) at atemperature of 77 K was 1,200,000 A/cm², and that of the superconductingfilm (C-4) was 600,000 A/cm².

TABLE 1 Evaluation of superconducting properties of superconductingfilms Critical current density Critical Jc (×10⁴ A/cm²) Super-conductingtemperature (@77 K, (@77 K, 1T, film Tc(K) OT) B// c-axis) (I-1) 90 500110 (C-1) 91 450 58 (I-2) 89.5 520 130 (C-2) 90.5 430 55 (I-3) 90.5 510100 (C-3) 91 400 45 (I-4) 90 480 120 (C-4) 90.5 450 60

As described above, the superconducting films of the invention showedcritical temperatures Tc equivalent to those of the conventionalsuperconducting films, and provided critical current densities equal toor more than those of the conventional films under a zero magneticfield. Furthermore, in a magnetic field of 1 T, the superconductingfilms of the invention showed critical current densities much higherthan those of the conventional superconducting films. Therefore, thesuperconducting films of the invention can allow larger currents to flowwhen they operate under the influence of a magnetic field, and aresuitable not only as devices to operate in such an environment, but alsofor applications such as cables, magnets, shields, current limiters,microwave devices, intermediate devices of these articles.

The present invention has been described in detail with respect tovarious embodiments, and it will now be apparent from the foregoing tothose skilled in the art that changes and modifications may be madewithout departing from the invention in its broader aspects, and it isthe intention, therefore, in the appended claims to cover all suchchanges and modifications as fall within the true spirit of theinvention.

1. A superconducting film comprising: a substrate having a surface; asuperconductor layer formed on said surface of the substrate; nanogrooves formed in said substrate, the nano grooves having a definedwidth and defining a current flowing direction when current flowsthrough the superconductor layer; rows of defect inducing parts formedin the nano grooves; and two-dimensional crystal defects in thesuperconductor layer on the defect inducing parts.
 2. Thesuperconducting film as claimed in claim 1, wherein each of saidtwo-dimensional crystal defects is a two-dimensional crystal defectwhich is continuous in a current flowing direction.
 3. Thesuperconducting film as claimed in claim 1, wherein each of saidtwo-dimensional crystal defects is an array of discontinuoustwo-dimensional crystal defects.
 4. The superconducting film as claimedin claim 1, wherein said two-dimensional crystal defects are irregularlydistributed on the substrate.
 5. The superconducting film as claimed inclaim 1, wherein said two-dimensional crystal defects are crystal grainboundaries, dislocation arrays, amorphous bodies formed from elementsconstituting said superconductor layer, nonsuperconductors or lowcritical temperature superconductors.
 6. The superconducting film asclaimed in claim 1, wherein said nano grooves have a width of not morethan 100 nm and a depth of not more than 100 nm and that the averagecenter to center distance of adjacent nano grooves in a directionperpendicular to a current flowing direction is not more than 500 nm. 7.The superconducting film as claimed in claim 1, wherein said substrateis a substrate of an oxide having a perovskite type crystal structure, arock-salt type crystal structure, a spinel type crystal structure, anyttrium stabilized zirconia type structure, a fluorite type crystalstructure, a rare earth C type crystal structure, or a pyrochlore typecrystal structure; or an oxide substrate, a nitride substrate, asemiconductor substrate, a nickel-based alloy substrate, a copper-basedalloy substrate or an iron-based alloy substrate on the surface of whicha buffer layer is formed from said oxide or boride.
 8. Thesuperconducting film as claimed in claim 1, wherein said superconductorlayer is formed from a superconducting material selected from the groupconsisting of copper oxide-based high temperature superconductingmaterials having the chemical formula LnBa₂Cu₃O_(7+x), where Ln is oneor more elements selected from the group consisting of Y element andrare earth elements and −0.5<x<0.2; copper oxide-based high temperaturesuperconducting materials having the chemical formula(Bi_(1−x)Pb_(x))₂Sr₂Ca_(n-1)Cu_(n)O_(2n+4+y), where 0<x<0.4, −0.5<y<0.5and n=1, 2 or 3; and superconducting materials which contain thechemical formula MgB₂ as a main component.
 9. The superconducting filmas claimed in claim 1, wherein said defect inducing parts are formedfrom a metal, an intermetallic compound, a nitride or an oxide.
 10. Thesuperconducting film as claimed in claim 1, wherein said superconductorlayer is formed from a plurality of layers and nano grooves are formedin each of said plurality of layers except a top layer.
 11. Asuperconducting film comprising: a substrate having a surface; asuperconductor layer formed on said surface of the substrate; rows ofnano holes formed in said substrate, each of the nano holes having adefined diameter, wherein the nano holes define a current flowingdirection when current flows through the superconductor layer; rows ofdefect inducing parts formed in said nano holes; and rows ofone-dimensional crystal defects in the superconductor layer of saiddefect inducing parts.
 12. The superconducting film as claimed in claim11, wherein each of said rows of one-dimensional crystal defects is arow of one-dimensional crystal defects which is continuous in a currentflowing direction.
 13. The superconducting film as claimed in claim 11,wherein each of said rows of one-dimensional crystal defects is an arrayof discontinuous rows of one-dimensional crystal defects.
 14. Thesuperconducting film as claimed in claim 11, wherein said rows ofone-dimensional crystal defects are irregularly distributed on thesubstrate.
 15. The superconducting film as claimed in claim 11, whereinsaid one-dimensional crystal defects are crystal grain boundaries,dislocation arrays, amorphous bodies formed from elements constitutingsaid superconductor layer, nonsuperconductors or low criticaltemperature superconductors.
 16. The superconducting film as claimed inclaim 11, wherein said nano holes have a diameter of not more than 100nm and that the average center to center distance of adjacent rows ofnano holes in a direction perpendicular to a flowing current is not morethan 500 nm.
 17. The superconducting film as claimed in claim 11,wherein said substrate is a substrate of an oxide having a perovskitetype crystal structure, a rock-salt type crystal structure, a spineltype crystal structure, an yttrium stabilized zirconia type structure, afluorite type crystal structure, a rare earth C type crystal structure,or a pyrochlore type crystal structure; or an oxide substrate, a nitridesubstrate, a semiconductor substrate, a nickel-based alloy substrate, acopper-based alloy substrate or an iron-based alloy substrate on thesurface of which a buffer layer is formed from said oxide or boride. 18.The superconducting film as claimed in claim 11, wherein saidsuperconductor layer is formed from a superconducting material selectedfrom the group consisting of copper oxide-based high temperaturesuperconducting materials having a chemical formula of LnBa₂Cu₃O_(7+x),where Ln is one or more elements selected from the group consisting of Yelement and rare earth elements and −0.5<x<0.2; copper oxide-based hightemperature superconducting materials having the chemical formula of(Bi_(1-x)Pb_(x))₂Sr₂Ca_(n-1)Cu_(n)O_(2n+4+y), where 0<x<0.4, −0.5<y<0.5and n=1, 2 or 3; and superconducting materials which contain MgB₂ as amain component.
 19. The superconducting film as claimed in claim 11,wherein said defect inducing parts are formed from a metal, anintermetallic compound, a nitride or an oxide.
 20. The superconductingfilm as claimed in claim 11, wherein said superconductor layer is formedfrom a plurality of layers and rows of nano holes are formed in each ofsaid plurality of layers except a top layer.